1. Field of the Invention
The present invention relates to the art of Fourdrinier papermaking. More specifically, the present invention relates to consistency control of the papermaking stock pond carried by a papermachine forming wire between the headbox and the couch roll.
2. Description of the Prior Art
The term "consistency" or "stock consistency" is used in the papermaking art to describe the relative fiber content in a given stock quantity. Thus, an increase in consistency of the pulp stock indicates a relative increase in the dry, wood fiber constituent of a slurried, fiber-in-water suspension.
Typically, Fourdrinier papermaking stock flows as a 0.5% to 2% consistency fluid jet from a headbox slice opening onto the upper surface of a moving, endless screen belt characterized as a Fourdrinier wire or fabric. This wire or Fabric, which is made from woven metal or plastic strands, is supported by a breast roll and a couch roll, the breast roll being located adjacent the headbox at one end of what is referred to as the forming section of the papermachine. Between the breast roll and the couch roll, the wire is supported by a multiplicity of table rolls and/or foils, and passes over suction boxes in its travel from the breast roll to the couch roll, each of these items being situated beneath an upper run of the wire at locations spaced between the breast and couch rolls. As the wire travels from the breast roll to the couch roll, water is drawn through the wire from a "pond" of wire supported pulp stock leaving a thin web formation of self-supporting, matted fibers on the upper surface of the wire. This web of matted fibers, still containing a considerable quantity of water, is lifted from the wire at the couch roll. After passing around the couch roll, the wire course returns through a series of return rolls to the upstream end of the forming section of the papermachine, where it passes around the breast roll and directly under the headbox to complete its path of travel.
At the end of the papermachine forming section, the proportion of fibers and solids in the wet, wire carried web is generally in the order of 15% to 22%. Here, the wet web of paper is peeled from the wire and guided into the papermachine pressing and drying sections wherein most of the remaining water is removed.
The rate at which water is removed from the pond is critical to the runnability of the papermachine and to the quality of paper produced. If the consistency is not sufficiently high by the end of the machine forming section, the fragile web will separate at the leap from the couch into the press section thereby disrupting the web production continuity. On the other hand, if the pond consistency is too high at the point of engaging top forming appliances such as a dandy roll or shear roll, such appliances are ineffective.
The prior art has manually judged pond consistency and, hence, pond drainage rate, by the physical location of a visually discernable "dry line" whereat the pond loses its light reflective surface sheen. At this point, the pond consistency is about 11% to 12%. Movement of the dry line up or down the machine direction signifies an exponential change in the pond drainage rate. Control over the dry line location has been asserted by manual adjustment of table suction boxes.
Due to many and complex reasons originating in the pulp mill, stock preparation and even in the raw wood supply, drainage characteristics of the pulp stock are continuously but irregularly changing. Consequently, pond consistency upstream of the dry line may vary by 100% at any given point over an interval of 30 minutes. These consistency variations are not manually perceptible to the papermachine tender but have dramatic influence over the effectiveness of top forming appliances.
It is an objective of the present invention, therefore, to provide a method and means for on-line consistency measurement of papermachine pond stock along the machine table.
Another object of the present invention is to provide a method and means for stabilizing the papermachine table pond drainage rate.
Another object of the present invention is to provide a method and means for regulating draft of Fourdrinier table suction boxes in response to stock pond consistency variations.
Another object of the present invention is to control papermachine stock pond consistency upon approach to a top forming appliance.
Another object of the present invention is to stabilize papermachine stock pond consistency at the couch roll.